Optical media such as compact discs (CDs) and digital versatile discs (DVDs) store data that is read back optically. Optical discs typically include a substrate that is made of plastic and an alternating reflective/non-reflective layer that includes a continuous spiral track for encoding data. An optical media playback device passes a laser over the track to read the data. An optical sensor receives light that is reflected back from the encoded data on the track.
Referring now to FIG. 1, an exemplary one-time recordable disc (e.g. a CD-recordable disc/CD-R and/or a DVD-recordable disc/DVD+/−R) 10 typically includes a polycarbonate plastic substrate 12, a dye layer 14, and a reflective metal layer 16. For example, the reflective metal layer 16 may include an aluminum layer. When the disc 10 is blank, the dye layer 14 is translucent, and light shines through and reflects off the reflective metal layer 16. When writing to the disc 10 with a laser, selected portions of the dye layer 14 are heated at a particular intensity and frequency, which turns the selected portions opaque. The opaque portions of the dye layer 14 do not reflect light. The opaque, non-reflective portions of the dye layer 14 are referred to as “marks.” Conversely, the translucent, reflective portions of the dye layer 14 are referred to as “spaces.” The optical media playback device may include a read laser to read data from the disc 10 and a write laser to alter the dye layer 14 for recording purposes.
During read back, the optical disc is rotated by the optical media playback device, which typically includes at least one laser, a spindle motor, and an optical sensor. The spindle motor rotates the optical medium. The laser is directed onto the tracks of the optical medium and the optical sensor measures reflected light. When the optical sensor generates a high current level corresponding to high reflectivity (i.e. a space), the data may be interpreted as a “1” (or “0”). When the optical sensor generates a low current level corresponding to low reflectivity (i. e. a mark), the data may be interpreted as a “0” (or “1”). In some devices, the space/mark signal, or commonly described as converted non-return to zero inverted (NRZI) signal, may be converted to a non-return to zero (NRZ) signal, as shown in FIG. 2, where 1's represent transitions and 0's represent the absence of transitions.
Referring now to FIG. 3, an exemplary rewritable disc (e.g. a CD-rewritable disc/CD-RW and or a DVD-rewritable disc/DVD+/−RW) 20 typically includes a polycarbonate plastic substrate 22, dielectric layers 24, a phase change compound layer 26, and a reflective metal layer 28. The phase change compound layer 26, for example, may be a chemical compound of silver, antimony, tellurium, and/or indium. A laser is used to heat the compound above a crystallization temperature and/or a melting temperature. When cooled rapidly from above the melting temperature, the compound remains in a fluid, amorphous state, and results in a non-reflective portion (i.e. a mark). Conversely, when maintained at the crystallization temperature for a certain length of time, the compound returns to a solid state before cooling down, and results in a reflective portion (i.e. a space). Therefore, in addition to a read laser, the optical media playback device may include a write laser that is powerful enough to heat the compound above the melting temperature and an erase laser that is powerful enough to heat the compound above the crystallization temperature.
In either one-time recordable disc or rewritable disc applications, the write process is non-linear. The marks (and spaces, in the rewritable disc application) are created according to focused heat from the write and/or erase laser. As such, heat diffusion and phase-change problems may occur at high rotational speeds of the moving optical media.
Referring now to FIGS. 4A through 4D, exemplary laser power profiles 30 required for writing to one-time recordable media, such as DVD+/−R media, are shown. To form a mark 32 beginning at a track position t, a pulse 34 begins at a track position t-y. To end the mark 32 at a track position u, the pulse 34 is terminated at a track position u-x. In other words, adjustment of the laser power 30 is offset from a desired position of the mark 32 as a result of heat diffusion non-linearity. The position offsets y and x (corresponding to pulse timing edges 36 and 38, respectively) may depend on various factors, including, but not limited to, the laser power 30, the media type, and/or the write speed.
Referring now to FIGS. 5A and 5B, typical laser power profiles 40 required for writing to rewritable media, such as DVD+/−RW media, are shown. Write pulses 42 are greater than both a cooling power level 44 and an erase power level 46. Similar to the laser profiles 30 as shown in FIGS. 4A through 4D, the pulses 42 begin prior to a desired initial position t of a mark 48 and terminate prior to a desired end position u of the mark 48.
Because of the non-linear nature of heat-based writing to optical media, the various power levels and pulse timing edges must be calibrated according to individual desired mark lengths (i.e. on a per mark-length basis). Further, the calibration is dependent on leading and trailing spaces adjacent to and on either side of the mark. In encoding schemes used in current optical media recording standards, both the marks and spaces may range from 3T to 14T in length, where T is a channel bit period.
A write strategy table located on the optical drive stores data entries specifying how a particular mark length should be formed. In other words, because the recording process is non-linear, the write strategy table includes the laser power, pulse timing edges, and other relevant data for each mark length. Typically, the write strategy table is stored in a nonvolatile memory module, such as a flash memory module, that is located on the optical drive.
Constructing the write strategy table typically requires a calibration sequence that is dependent on both media type (e.g. the manufacturer of the media) and the write speed. The calibration sequence is conducted during manufacturing for each optical media playback device. Further, the calibration sequence includes data to account for all possible known media types that are available to a potential user of the optical media playback device and is stored on the nonvolatile memory module.
When new media types are introduced by media manufacturers, however, the write strategy table must be updated. For example, a user may download (e.g. from the Internet) new firmware that includes an updated write strategy table from the manufacturer. When Internet access is not available, the user may need to acquire an update disk from the manufacturer via mail. In certain circumstances, a user may inadvertently obtain counterfeit or off-specification media that can not be properly written with the original write strategy table. Therefore, no amount of updating from the drive manufacturer can solve the resulting write problems. Often, consumers purchase optical media playback devices such as DVD drives and attempt to operate the drives with low-cost media. When the consumers discover that the drives are not able to write or read the media properly, many return the drive the store, resulting in increased costs to drive manufacturers.